High resolution liquid crystal displays (LCD), used in conjunction with overhead projectors, have become popular devices with which to project text, graphics and images from personal computers to a large audience. Most LCD projection panels are designed to operate with overhead projectors that are characterized as "transmissive," that is to say, light rays emanating from a projector lamp in the base of the projector, under the LCD, pass through the LCD once before being projected onto a screen. In reflective overhead projectors, however, the lamp is in the "head" of the projector, over the LCD, and the light rays pass down through the LCD and are then reflected back up through the LCD before being projected onto a screen. Reflective overhead projectors are advantageous because they can be condensed into a smaller volume, and thus can be made highly portable. For example, some reflective overhead projectors can fit into a briefcase-like carrying case. When LCD projection panels that have been designed for use with transmissive overhead projectors are used with reflective overhead projectors, the result is usually a very dark, double image.
One reflective overhead projection system includes an LCD having polarizers on both sides. This sandwich is separated by an air gap from a fresnel lens and a reflector. This system has three drawbacks. First, the projected image is a double image because the plane of the image is separated quite some distance from the plane of the reflection. Second, the system suffers from low light transmission--less than 10% for a super twisted nematic LCD. Third, the projected image suffers from glare from the specular reflection of the projector light from the top of the polarizer surface, which obscures the desired image.
Another reflective LCD overhead projection system is shown in Japanese Kokai 2-193,183 published on Jul. 30, 1990. That system uses an LCD pressed against a polarizing plate separated by an air gap from a minor fresnel lens. Light rays from a projector lamp first pass through the LCD, the polarizing plate, and then the air gap. The light rays are then reflected by the minor fresnel lens, and pass back through the polarizing plate and exit the LCD. While this system eliminates the double image problem, the double pass of the light rays through the polarizing plate and the absorption attendant each pass limits the overall brightness achievable with this system, as does absorption by the metallized surface of the fresnel lens.